An MRI apparatus is a medical image diagnostic apparatus that applies a high-frequency magnetic field and a gradient magnetic field to an object placed in a static magnetic field and measures a signal generated from the object by nuclear magnetic resonance to visualize to image the object. In an MRI apparatus, a slice gradient magnetic field that generally identifies an imaging surface is applied at the same time when an excitation pulse (high-frequency magnetic field pulse) that excites magnetization in the surface is provided to obtain a nuclear magnetic resonance signal (echo) to be generated in a stage where the excited magnetization converges. At this point, in order to provide positional information to the magnetization, a phase encoding gradient magnetic field in a mutually vertical direction in the imaging surface and a read-out gradient magnetic field are applied while the echo is obtained after the excitation. The measured echo is placed in a k-space whose horizontal axis is kx and the vertical axis is ky to reconstruct an image by performing inverse Fourier transform for the echo in the k-space.
A pixel value of a reconstruction image is a complex value including absolute value information and phase information. The absolute value and the phase are determined by an imaging sequence type, an imaging parameter comprised of a pixel size, a repetition time, etc., a magnetization density in an object and the relaxation time, spatial distribution of a resonance frequency, etc. As a normal reconstruction image, although there are many cases where a gray-scale image whose absolute value is a pixel value is used, a method to generate an image by combining an absolute value with a phase.
For example, the patent literature 1 discloses that a phase of each pixel of a reconstruction image is converted into a value whose value range is [−π,π] to generate a phase image, additionally, a phase mask where a value range of the phase image was converted into [0, 1] is created, and a product of a value which is a q-th power (q≧1) of each pixel phase of the phase mask and an absolute value of the same pixel is calculated to generate an image whose pixel value is the calculated product. The value q is determined so that the contrast-noise ratio becomes the maximum. A high contrast image can be obtained by this process.
On the other hand, the patent literature 2 discloses that static magnetic field intensity distribution is calculated from two phase distribution images whose echo time was shifted by Δt using that static magnetic field intensity information is reflected to a phase value of the echo. The static magnetic field intensity distribution is used to perform shimming.
Also, the non-patent literature 1 discloses that phase correction is performed for an echo signal in a k-space using the echo signal obtained in zero-encoding. A process to delete phase information caused by static magnetic field inhomogeneity by performing a high-pass filter process or low-pass filter process for an echo in the k-space is also known.